Originally Posted by Evshrug
Hey ClieOS, how do you measure output impedance? If I need special equipment, do you think an audio gear or guitar specialist shop would be able to measure an amp for me? Also, with the usual recommendation for output impedance to be 1/8th the ohms of the headphone impedance, what is the effect/benefit of impedance adapters such as the one to make an Etymotic ER4S "into" an ER4P (or do I have the two mixed up?)?
Thanks for sharing your expertise!
To do basic output impedance measurement, you will need
(1) A digital multimeter that can measure AC in mV.
(2) A fixed load that you can plug into the amp. I soldered a pair of matched 47ohm resistor to a Neutrik jack for this.
(3) An interconnecting cable. This is for you to measure the unload volatge easily straight from the plug
(4) A tone generator. Any decent USB DAC or sound card with Audacity on the PC will do.
(5) Some math calculation.
You can find the detail here
, and it is basically the same thing NwAvGuy has described in his article about output impedance. He uses 100Hz mostly but I choose 1000Hz since it is more widely used. Note that many amp have higher output impedance on the top treble (15kHz~20kHz) so you don't want to measure in those area. Given what need to have, I don't think most typical shops will have the needed fixed load around for output impedance measurement unless they are also doing repair work and has some spare resistors around to play with.
Anyway, 1/8th is the minimum rule most are following. Some even go for the tougher 1/10th rule. The impedance adapter on ER4S (100ohm, 4P is 27ohm) is mainly to reshaped the FR curve to match the Etymotic 'ideal' curve and has less to do with dampening or the 1/8th rule. But in a way, you can say 4P is the colored sounding version of the 4S - it is just a matter of how you interpret words.